Building guards stopped checking identification cards at the Department of Agriculture yesterday. Nobody was really quite sure why, but the rumor was that Agriculture Secretary Bob Bergland's wife, Helen, was hassled by the guards and couldn't get in.

Tom Sand, a spokesman for Bergland, allowed that on one occasion the Secretary's wife did have trouble getting into the building after hours, but he insisted that was not the reason for dropping the ID check.

The real reason: "The Secretary, the deputy secretary and several others at the top level have had trouble getting in and out because of what we affectionately call the rent-a-cops," Sand said.

"The guards were hired to prevent pilferage," Sand continued. "I don't see how ID cards are preventing pilferage. A Social Security card is enough to get in. We see no evidence at all of increased security - no reduction at all in pilferage."

Officials of the ENSEC Service Corp. of Timonium, Md., which supplies the guards for the Agriculture Department under a General Services Administration contract, could not be reached for comment.

One guardd at the department's south building on Independence Avenue at 14th Street SW said he was disturbed by the fact that the checks had been dropped. "Anyone could get into the building," he said. He declined to be identified by name.

Most employees interviewed at the entrance to the south building said they were unmoved by the change.